Neural stimulation has been applied to modulate various physiologic functions and treat various diseases. One example is the modulation of cardiac functions using autonomic modulation therapy (AMT) such as vagus nerve stimulation (VNS) therapy in a patient suffering heart failure or myocardial infarction. The myocardium is innervated with sympathetic and parasympathetic nerves including the cardiac branches of the vagus nerve. Activities in the vagus nerve, including artificially applied electrical stimuli, modulate the heart rate and contractility (strength of the myocardial contractions). Electrical stimulation applied to the vagus nerve is known to decrease the heart rate and the contractility, lengthening the systolic phase of a cardiac cycle, and shortening the diastolic phase of the cardiac cycle. This ability of VNS is utilized, for example, to control myocardial remodeling.
Batteries are used as energy sources for implantable medical devices including those delivering neural stimulation. While the use of a battery allows a medical device to be totally implantable, without the need of transcutaneous power transmission, the power consumption and longevity of the medical device is limited by the capacity of the battery. For example, many implantable medical devices providing for cardiac and/or neural stimulation treating cardiac disorders are long-term treatments that may last up to the patient's lifetime. When the battery of such an implantable medical device is no longer able to provide sufficient energy for the operation of the device, the device is to be explanted and replaced. Because it may take weeks to months to arrange for the device replacement after such need is indicated based on the energy state of the battery, there is a need to manage behavior of the implantable medical device during this period of time, when the battery is near its end of life.